The present invention relates to a vacuum power interrupter, and more particularly to a vacuum power interrupter wherein a vacuum vessel is constituted so that the opening end of a bell shaped metal casing is fitted over an insulating end plate.
Generally, a vacuum power interrupter, as shown in FIG. 1, comprises a vacuum vessel 4 consisting of an insulating envelope 1 of glass or ceramic, end plates 2 and 3 attached to both axial ends of the insulating envelope 1, and stationary and movable contact rods 7 and 8 having electrical contacts 5 and 6 on the respective ends disposed so that the latter is in contact with the former or away from the former within the vacuum vessel 4.
Recently, it has been intended to increase an interrupting capacity of a vacuum power interrupter. To meet with this requirement, the radius of the electrical contact is enlarged, resulting in enlarging the radius of the insulating envelope. However, it is difficult to fabricate the insulating envelope of glass or ceramic so that the accuracy of the dimension thereof is maintained within a predetermined region. Particularly, it is very difficult to sufficiently satisfy such a requirement with respect to an insulating envelope of which radius is large. If an attempt is made to fabricate such a large sized vacuum interrupter, the material constituting a vacuum vessel becomes very expensive. As a result, the cost of the vacuum power interrupter also becomes expensive.
In view of this, another type of a vacuum power interrupter is proposed as shown in FIG. 2. This type of the vacuum power interrupter comprises a vacuum vessel 12 consisting of an insulating end plate 9 of ceramic having a ring shaped projection 10 integrally formed thereon, and a metallic cup member 11 of which the opening thereof is fastened to the ring shaped projection 10 and brazed thereto. A movable contact rod 14 having an electrical contact 13 is movably in the axial direction of the vacuum vessel supported by the bottom portion 11a of the cup member 11 constituting the vacuum vessel 12. A bellows 15 is mounted on the cup member 11 so that the upper end thereof is hermetically brazed to the movable contact rod 14 while the lower end thereof is hermetically brazed to the bottom portion 11a of the cup member 11. A stationary contact rod 17 having an electrical contact 16 in contact with the contact 13 or away therefrom is inserted into the insulating end plate 9 and is hermetically brazed thereto. A cup-shaped arc-shield member 18 for preventing a stain or deterioration of the insulating end plate 9 due to a metal vapour produced when the electrical contact 13 is in contact with the electrical contact 16 or away therefrom is provided so as to surround electrical contacts 13 and 16.
With the above mentioned vacuum power interrupter shown in FIG. 2, since one end plate 9 of insulator is solely required as compaired with FIG. 1 assembly, it is possible to reduce the cost for material constituting the end plate. However, in regard to FIG. 2 assembly, in order to obtain an insulating creeping distance of the insulating plate 9 within the vacuum vessel 12 and suppress that the vacuum vessel 12 is enlarged, with the resultant that the radius of the cup member 11 is enlarged, there is provided the annular projection 10 on the insulating plate 9. In order to fabricate the annular projection 10 of the insulating pipe 9 of ceramic, it is extremely difficult to form a mold, such as a metallic pattern or pressed pattern.
Also, another drawback is pointed out that many working steps are required for correcting deformation due to contraction of ceramic being produced when molding and sintering is effected. As a result, this results in high cost of the insulating plate 9. Therefore, the vacuum power interrupter becomes expensive. Since the cup member 11 is mounted on the insulating end plate 9 so that the opening end of the cup member 11 is in contact with the upper end of the annular projection 10 of the insulating end plate 9 and brazed thereto. As a result, it is extremely difficult to become uniform the distance between the cup member 11 and the arc-shield member 18 when fabricating the vacuum vessel. As a result, a positioning tool for positioning or aligning the movable contact rod 14 with respect to the stationary contact rods 17 is required. Otherwise, it is necessary to enlarge the outer radius of the cup member so that the aligning displacement is allowed.
In general, when connecting ceramic to metal by brazing method, in view of the difference of the coefficient of thermal expansion therebetween, it is desirable that the joining or connecting surface is as small as possible. On the contrary, in view of sealing and mechanical strength at the time of joining, it is desirable that the joining or connecting surface is as large as possible. However, in the above mentioned vacuum power interrupter shown in FIG. 2, the opening end of the cup member 11 is connected to the annular projection 10 and the cup member 11 is formed with cup-shaped durable metalic rigid body. Accordingly, it is impossible to absorb or relax a thermal stress produced at the time of connection by means of brazing material due to the deformation of the cup member. As a result, there is possibility that the insulating plate 9 of ceramic may be broken. Further, another drawback is pointed out as follows: The cup member 11 is made of rigid body. The weak or fragile portion in strength is in the vicinity of its opening peripheral. This portion is connected to the insulating end plate 9 of ceramic, thereby making it possible to strengthen the durability. However, in the cup member constituting a vacuum vessel, an internal stress is apt to be concentrated in the vicinity of its opening peripheral. In this portion, there remains a therml stress at the time of joining to the insulating end plate.
As a result, this portion is considered as a most fragile part. Accordingly, when an impact produced at the time of energizing and interrupting operation is applied to this portion, there is possibility that the joining portion or the insulating plate 9 will be broken.
Furthermore, in the above mentioned vacuum power interrupter, the bellows 15 is mounted on the upper surface of the cup member 11 so that the one end thereof is fixed to the bottom portion 11a of the cup member 11. While the other end thereof is fixed to the circumferential surface of the movable contact rod 14. Accordingly, there is problem that the bellows is apt to be damaged at the time of assembling or connecting to an actuating unit or during operation.
A bottom portion 18a of the arc-shield member 18 is interposed between the electrical contact 16 and the insulating end plate 9, and is directly brazed thereto. The electrical contact 16, the arc-shield member 18, and an insulating end plate 9 of which coefficients of thermal expansion are different each other are joined in a relatively large contact surface. Accordingly, when energizing, each member is always heated.
Accordingly, the electrical contact 16, the arc-shield member 18 and the insulating end plate 9 are subject to thermal stress due to the difference of coefficient of thermal expansion therebetween. For this reason, there is possibility that the insulating end plate 9 of ceramic which is brittle material in view of mechanical strength will be broken. Since thermal conductivity of ceramic forming the insulating end plate 9 is far smaller than that of metal, when a heat produced by arc at the time of interruption transfers to the insulating end plate 9 through the electrical contact 16, and the bottom portion 18a of the arc-shield member 18, there is possibility that there occurs cracks due to heat on the joining or connecting surface facing to the bottom portion 18a of the arc-shield member 18. Further, it is pointed out that the occurrence of the cracks is promoted when an impact due to energization and interruption is applied to the aforesaid joining surface.
Another prior art vacuum power interrupter matured or laid open to public inspection are as follows
U.K. Patent Specification No. 1,298,448 published on Dec. 6, 1972 discloses a vacuum vessel comprising a cylindrical metal casing, and an insulating end plate fitted into the axial end of the vacuum vessel. U.S. Pat. No. 3,727,018 patented on Apr. 10, 1973 discloses a vacuum vessel comprising a cylindrical casing of Fe-Ni-CO (KOVAR), and insulating end plates fitted into the both axial ends of the casing wherein a movable contact rod is supported by the insulating end plate through the bottom portion of a bellows. U.K. patent application GB 2010587A published on Jun. 27, 1979 discloses a vacuum vessel comprising a cylindrical metal casing, and insulating end plates fitted into the axial end of the metal casing.
In these vacuum power interrupter, it is clear that the same drawbacks mentioned in regard to the vacuum power interrupter shown in FIG. 2 are pointed out without detailed discussion.